Z3 theory plug-in error - z3

Question

I have created a custom theory plugin, which does nothing at the moment. The callbacks are all implemented and registered, but they simply return. Then, I read in a bunch of declare-consts, declare-funs, and asserts using Z3_parse_smtlib2_string, and pass the resulting ast to Z3_assert_cnstr. A subsequent call to Z3_check_and_get_model fails with the following error:
The mk_fresh_ext_data callback was not set for user theory, you must use Z3_theory_set_mk_fresh_ext_data_callback
As far as I can tell, Z3_theory_set_mk_fresh_ext_data_callback does not exist.
Using the same string, but without registering the theory plugin, Z3_check_and_get_model returns sat and gives a model as expected.
I am using version 4 and the Linux 64 bit libraries.
The full example is here: http://pastebin.com/hLJ8hFf1

Answer 1

The problem is the model-based quantifier instantiation module (MBQI). This module tries to create a copy of the main logical engine. To create the copy, Z3 must copy every theory plugin. It can do it for all builtin theories, but not for external theories.
The original theory plugin API did not have support for copying itself because it was implemented before the MBQI module. The API Z3_theory_set_mk_fresh_ext_data_callback is meant for that. However, it was not exposed yet for several reasons.
The main issue is that Z3 4.0 has a new API for solvers. The current theory plugin API is incompatible with the new solver API.
We are investigating ways of integrating them.
In Z3 4.0, the theory plugins only work with the old (deprecated) solver API.
To avoid the problem you described, you just have to disable the MBQI module. You can do that by setting MBQI=false when creating Z3_context.
In C, you can do that using the following code fragment.
Z3_config cfg;
Z3_context ctx;
cfg = Z3_mk_config();
Z3_set_param_value(cfg, "MBQI", "false");
ctx = Z3_mk_context(cfg);
This also explains why your plugin works on quantifier-free formulas. The MBQI module is not used for this kind of formula.